The invention relates to an improved method for removing residual vinyl chloride monomer from aqueous polyvinyl chloride resin mixtures.
The preferred methods for preparing polyvinyl chloride comprise carrying out the polymerization of vinyl chloride in an aqueous emulsion or suspension. The aqueous resin mixture obtained from the polymerization reactor generally contains from about 10 to about 60% by weight polyvinyl chloride and from about 0.5 to about 10% by weight residual vinyl chloride monomer. The vinyl chloride monomer tends to remain with the polyvinyl chloride during processing of the material and is released when the resin is dried or heated during the processing.
A major portion of the vinyl chloride monomer can be removed from the polyvinyl chloride during the drying operation. However, the vinyl chloride monomer is lost with the gases from the dryer which increases the cost of producing polyvinyl chloride. In addition, the vinyl chloride monomer in the gases from the drier can contaminate the environment in the vicinity of the drier.
In order to improve the efficiency of vinyl chloride utilization in the process and to reduce the amount of vinyl chloride introduced into the environment, attempts have been made to lower the concentration of vinyl chloride monomer in the aqueous resin mixture before processing.
It has been suggested that the monomer can be separated from the polyvinyl chloride by releasing the pressure on the aqueous resin mixture in the reactor and stripping at reduced pressure or by steam distillation. These methods are not commercially useful when the aqueous resin mixture is a latex due to the foaming of the latex and the difficulty in achieving low levels of vinyl chloride monomer in the latex. Even with a suspension resin mixture it is difficult to achieve low levels of vinyl chloride monomer in the aqueous resin mixture.
U.S. Pat. No. 3,052,663 discloses a method for separating unreacted vinyl chloride monomer from polyvinyl chloride latex by contacting the latex with an aliphatic hydrocarbon. The process requires many difficult contacting and filtration steps to separate the polyvinyl chloride from the hydrocarbon phase. The method would not be suitable where the resin is to be spray dried to form a white, small particle size, powdery product.
Polish Pat. No. 56,169 discloses that monomer can be removed from latex and foaming can be controlled by passing the polyvinyl chloride latex over an inclined surface of varying slope. The amounts of vinyl chloride monomer remaining in the latex is not disclosed in the patent.
It is an object of the present invention to provide an effective method for removing vinyl chloride monomer from aqueous resin mixtures of polyvinyl chloride. It is an object of the invention to provide an effective method of removing vinyl chloride monomer from an aqueous suspersion resin. It is a further object of the invention to provide a method for controlling foaming and to minimize flocculation due to drying of foams when removing vinyl chloride monomer from polyvinyl chloride latex. It is a further object of the invention to provide a method by which a latex containing less than 400 parts per million vinyl chloride monomer can be prepared.